This invention relates to security documents, such as banknotes or the like, and is particularly concerned with providing a security document which includes means for verifying the security document or another similar document.
A wide variety of security devices or features for security documents, such as banknotes, travellers cheques or the like has been proposed previously. Examples of such security devices and features include: optically variable devices, such as holograms and diffraction gratings; security threads or strips; microprint; fine line or xe2x80x9cfiligreexe2x80x9d patterns; Moire inducing patterns; and fluorescent inks, phosphorescent inks, pearlescent inks or other optically variable inks, such as metameric inks.
Metamerism has been described as xe2x80x9cthe property of the eye and brain to receive the same colour sensation (under specific lighting conditions) from two objects with different spectral energy distributionsxe2x80x9d. Metameric inks have the unique property of appearing to change colour when viewed in different lighting conditions. For example, two inks with different metameric properties may appear to be of an identical colour when viewed in a particular white light environment, say daylight, but when viewed in different lighting conditions, e.g. in incandescent light, or in filtered light, the two inks will appear to have different reflective colours, so that one ink is distinguishable from the other. The optical effect of inks with metameric properties is widely accepted as a security device which inhibits such counterfeiting attempts as computer scanning and colour photocopying. Colour photocopying and colour printing is typically restricted to four different pigments (black, cyan, yellow and magenta) when attempting to match the colour of the original. In the event of reproducing metamerism, the colour distinction of an image with a different colour appearance in a particular lighting environment, is not as evident in the copy when compared to the original. The use of metameric inks as an anti-counterfeiting feature or security device in security documents is also described in U.K. Patent No. GB 1407065.
One disadvantage of metameric inks as a security device is that they require an optical filter or other external aid, to provide the required lighting condition for verification of the security device. Other types of security devices also require external aids for their verification. For example, fluorescent inks may require a source of ultraviolet light for their verification, and microprint, fine line and filigree patterns may require a magnifying lens for verification. Also, Moire inducing patterns, which produce fringes or a Moire effect when there is interference with a superimposed similar pattern, have hitherto only been effective as an anti-counterfeiting device when an attempt is made to reproduce a security document by colour photocopying. Also, a separate viewing device is required to verify that a security document has a Moire inducing pattern.
In Australian Patent Specification No. AU-A-87665/82 there is disclosed a security document and a method of producing a security document, in which opacifying coatings of ink are applied to both sides of a sheet-like substrate formed from a clear plastics film. The security document may be produced with some areas to which no opacifying coating is applied on both sides of the clear plastics substrate. These clear, transparent areas are known as xe2x80x9cwindowsxe2x80x9d and are particularly suitable for incorporating security devices, for example diffraction gratings, optically variable devices and embossed images, which can be inspected in the transparent areas or windows from both sides of the security document.
The present invention proposes that a transparent window in a security document may be used as a means for verifying, enhancing or optically varying a security device elsewhere on the document or on another security document.
According to one aspect of the invention, there is provided a security document such as a banknote, comprising a single flexible sheet formed from a substrate bearing indicia, said sheet having a first portion of transparent plastics material, and a security device provided at a second portion of the sheet spaced laterally from the transparent first portion, wherein the transparent first portion includes self-verification means to verify or inspect the security device when the sheet is bent, folded or twisted to bring the first and second portions into register with one another.
In addition to verifying or inspecting a security device at a laterally spaced location on the same security document, the self-verifying means may also be used to verify or inspect a security device on another security document.
The security document is preferably formed from a sheet-like substrate of transparent plastics material to which at least one opacifying layer or coating is applied on one side or both sides of the substrate except in the area or areas where it is desired to provide a transparent, essentially indicia-free portion or xe2x80x9cwindowxe2x80x9d in the security document. The at least one opacifying layer therefore only partially covers the surface of the substrate to leave said first portion essentially indicia-free.
The opacifying layer or at least one of the opacifying layers on either side of the plastics substrate may comprise a paper layer which bears indicia. Alternatively, in a preferred embodiment, the opacifying layer on each side of the sheet comprises at least one coating of opacifying ink applied to each surface of a transparent plastics substrate. It is also conceivable that a security document in accordance with the invention could be formed almost entirely from an opaque paper or laminated substrate construction except for an area or areas formed from a transparent plastics material to provide a window or windows.
The security document may take any desired shape, but in the case of a banknote, cheque or the like the flexible sheet is preferably rectangular. In the case of a square or oblong rectangular sheet the first and second portions may be so disposed that folding of the sheet about a center line brings the first and second portions into register. For an oblong sheet having a major axis and a minor axis, the first and second portions may be so disposed that folding of the sheet about a line coincident with or parallel to either the major axis or the minor axis brings the first and second portions into register. Alternatively, the sheet may be folded about a line inclined to the major and minor axes, such as a diagonal line in a rectangular sheet, to bring the first and second portions into register.
Instead of folding the sheet, the flexible sheet may be bent or folded to form a cylinder to bring the first and second portions into register so that the security device in the second portion may be inspected or verified by viewing the security device through the self verification means in the first portion.
In one embodiment of the invention, the self-verification means comprises an optical lens provided in the transparent first portion or window and the security device provided at the second portion comprises a printed or embossed feature which can be inspected, enhanced or optically varied by viewing through the optical lens of the security document or through an optical lens of another, similar security document.
One type of optical lens which may be provided in the window of a security document in the present invention is a Fresnel magnifying lens of the type used in overhead projectors. Such a magnifying lens may be formed by embossing, engraving or otherwise deforming the transparent, indicia-free plastics portion with concentric circular lines. A magnifying lens may alternatively be produced by applying an ultraviolet (UV) or otherwise curable varnish or coating which is printed with the required structure which is then made permanent by the curing process. A magnifying lens provided in the window of a flexible security document may be used to enlarge microprinting, a small image or a fine line or filigree pattern on another part of the security document or on another, similar security document. As an alternative to the Fresnel magnifying lens, a multiple micro-lens array or a lenticular lens array may be used.
The self-verification means may comprise another form of optical lens, such as a distorting lens. A distorting lens may be used to distort a security device, feature or image on another part of the security document, or to correct a distorted feature or image on another part of the security document.
In another embodiment of the first aspect of the invention, the security device comprises an area printed with metameric inks and the self-verification means comprises an optical filter for viewing the area printed with metameric inks. The optical filter is preferably arranged to restrict the wavelength distribution of the light that is incident on, and/or reflected from the area printed with metameric inks. This may be achieved by providing a colour tinted optical filter in the transparent, essentially indicia-free portion. A colour tinted transparent window creates a restricted or altered wavelength environment so as to reveal the colour changing properties of an image printed in metameric inks enabling the authentication of the banknote to be verified.
In accordance with a second aspect of the invention, there is provided a security document comprising a flexible sheet formed from a substrate bearing indicia, said sheet having an essentially indicia-free portion of transparent plastics material, wherein the transparent, essentially indicia-free portion includes a colour tinted optical filter for viewing an area printed with metameric inks on the same or a different security document.
The optical filter in the transparent window may be produced by various processes. One process for producing a colour tinted optical filter is to include appropriate pigments with a polymer in the production of a plastics film substrate to achieve an overall tint of the plastics film. In an alternative process a tinted varnish may be applied over a transparent plastics window by a gravure or offset process.
In accordance with a third aspect of the invention, there is provided a security document comprising a flexible sheet formed from a substrate bearing indicia, said substrate having an essentially indicia-free window of transparent plastics material including self-verifying means having polarisation characteristics for verifying a security device in the form of a second transparent polarising window at another location on the same or a different security document.
Polarisation is an optical effect widely used in items such as polarised sunglasses. Light waves from a luminous source vibrate not only in the vertical and horizontal planes but all others in between. Polarisation is an effect whereby the light is confined to one direction only. In the event of the plane polarised light passing through a secondary polarising medium whose polarisation axis is at right angles to the first, then near zero intensity of the light results.
This phenomenon is utilised, in the present invention, by using the transparent windows of security documents such as polymer banknotes. By superimposing a clear window over a second window, both with plane polarisation characteristics, the polarisation property including light extinction will be observed. The second polarising window may be present on the same security document or may be present on a different security document. In each case, the polarisation effect is achieved from the combination of the transparent polarising windows.
When two transparent polarising windows are located at different locations on a single flexible security document, the first and second polarising windows are preferably constructed and arranged in such a manner that, when the flexible security document is folded over itself to bring the polarising windows into register, the second polarising window has a polarisation axis extending at an angle to the polarisation axis of the first polarising window so that the intensity of light transmitted through the windows is reduced. If the polarisation axes of the first and second polarising windows are substantially perpendicular to one another in the folded security document, the intensity of light transmitted through the windows will be nearly zero.
A banknote with polarising windows may be formed by various methods. In one possible method, a transparent plastics substrate or film may be stretched in one direction during manufacture. In other methods, liquid crystals may be incorporated in a transparent polymeric film which may form the substrate or be added as a coating to the substrate.
In another embodiment of the invention the self-verification means comprises a feature including a first set of lines and the security device comprises a feature including a second set of lines, wherein an interference effect is produced when the security document is bent or folded to bring the self-verifying means and the security device into register. Preferably, the self-verifying means and the security device are Moire inducing patterns.
In accordance with a fourth aspect of the invention, there is provided a security document comprising a flexible sheet formed from a substrate bearing indicia, said sheet having a first portion of transparent plastics material including self-verifying means in the form of a Moire inducing pattern for verifying a security device in the form of another Moire inducing pattern at another location on the same or a different security document.
Moire inducing patterns consist of sets of threads or fine lines which produce optically variable effects when a first Moire inducing pattern is superimposed on a second Moire inducing pattern in which the threads or fine lines are inclined at an angle to the threads or fine lines of the first Moire inducing pattern. The transmission of light through superimposed or overlaying sets of inclined lines produces the appearance of dark bands known as xe2x80x9cTalbot fringesxe2x80x9d which may form an image.
The use of Moire inducing patterns has been previously proposed in security documents as a security device or anti-counterfeiting feature to deter counterfeiting by photocopying. However, in such documents, the Moire effect or fringes are only apparent on the counterfeit photocopied image of a security document which includes a Moire inducing pattern. In the present invention, a Moire inducing pattern is incorporated into the transparent plastics window of a security document as a self-verifying security device which, together with another Moire inducing pattern provided at another location in the same security document or in another security document, produces a Moire effect which is readily identifiable to verify the document.
When first and second Moire inducing patterns are provided at two different transversely spaced locations in a single flexible security document, the first and second Moire inducing patterns are preferably arranged in such a manner that, when the flexible security document is folded over itself to bring the Moire inducing patterns into register, the set of lines of the second Moire inducing pattern are inclined to the set of lines of the first Moire inducing pattern.
The set of lines provided in a transparent window to form a Moire inducing pattern may be formed by any convenient printing, embossing or engraving process.
According to a further aspect of the invention, there is provided a method of verifying a security document in accordance with any of the preceding aspects of the invention, wherein the method comprises the step of bending, folding or twisting the flexible sheet to bring the first portion including the self-verifying means into register with the security device provided at the second portion of the sheet.